1. Technical Field
The invention concerns tuning an RF signal generator to a plasma-loaded RF signal applicator of a plasma reactor, such as an antenna or an electrode, by serving the RF frequency so as to optimize the value of a selected parameter, such as, for example, delivered current or voltage or power, using a fixed tuning circuit. The fixed tuning circuit may be switched to a selected one of several transformers, depending upon the tuning range anticipated.
2. Background Art
RF plasma reactors of the type employed in processing semiconductor wafers require a large amount of RF power to maintain a plasma within a vacuum chamber, typically on the order of a thousand watts at RF frequencies on the order of several megaHertz. To maintain a high density plasma, the RF power is best inductively coupled into the chamber via an overlying coil antenna, while plasma ion energy can be controlled by controlling the voltage on the semiconductor wafer being processed. Typically, the RF signal source is an RF generator having an output impedance of 50 .OMEGA. with substantially no reactance. The input impedance presented by the plasma-loaded coil antenna is typically not 50 .OMEGA. and has substantial reactance, so that there is a substantial impedance mismatch. For example, the coil antenna typically has a length much less than a quarter wavelength of the RF signal, so that the coil antenna presents an impedance having a real part much less than that of the RF generator (which is typically 50 .OMEGA.) and having a very high inductive reactance. Such a mismatch causes RF power to be wasted by reflection back to the RF generator rather than being delivered to the plasma, so that it is difficult to control the amount of RF power delivered to the plasma. As a result, process control is compromised. One solution to this problem is to provide a fixed RF match circuit having lumped reactive elements so as to maintain a zero phase angle between RF voltage and current. Moreover, optionally a transformer can be employed to provide a match between the output and input impedance magnitudes.
The problem with such a fixed match circuit is that the input impedance of the plasma-loaded coil antenna changes as process conditions inside the reactor chamber change. Thus, as changes in plasma conditions change the plasma-loaded antenna impedance, the match circuit no longer can perform its function, and RF power delivered to the plasma falls off. Such a reduction in delivered RF power typically distorts the plasma processing of the wafer and in many cases is unacceptable. Therefore, the best solution in the art is to provide an RF impedance matching apparatus that adjusts the impedance match in response to changes in the plasma-loaded impedance of the antenna.
A conventional plasma reactor having such a variable RF impedance match circuit is depicted in FIG. 1A. The plasma reactor includes a reactor chamber 100 evacuated by a pump 105, a wafer support pedestal 110 on which a wafer 115 may be placed, an overhead coil antenna 120, and a gas inlet 125 into the chamber coupled to a process gas supply 130. An RF plasma source signal generator 140 is connected through an RF impedance match box 150 while an RF bias signal generator 160 is connected through another RF impedance match box 170 to the wafer pedestal 110. The power applied by the plasma source signal generator 140 controls plasma ion density in the chamber 100 while the power applied by the bias signal generator 160 controls plasma ion energy near the wafer 115. In some cases, both ends of the coil 120 may be connected to ground through respective capacitors shown in dashed line in FIG. 1A.
The RF impedance match boxes 150 and 170 are generally the same and will be described with reference to the RF impedance match box 150. The impedance match is provided by a conventional "pi-network" consisting of a pair of parallel capacitors 180, 185 (which are really capacitor circuits) on either side of a series inductor 190. Each of the capacitor circuits 180, 185 is controlled by an impedance match controller 200. The controller 200 monitors the forward voltage, reverse voltage and current/voltage phase angle via a conventional directional coupler 210 at the RF input 150a and computes from these three parameters a correction to the capacitance of each variable capacitor circuit 180, 185, using a network model 220. The controller 200 issues control signals at its control outputs 200a, 200b to the variable capacitors 180, 185 to effect the needed corrections in their capacitance values. Each of the variable capacitors 180, 185 can be a mechanically variable capacitor or an electrically variable capacitor circuit as illustrated in the drawing, the latter choice being preferable. FIG. 1A illustrates one example of the latter case, in which each variable capacitor circuit 180, 185 consists of an electrically variable inductor 230 connected in parallel with a fixed capacitor 240. The variable inductor 230 is a saturable reactor consisting of a primary winding 232, a magnetically permeable core 234 and a smaller control winding 236 connected to a variable current source 238. A respective one of the control outputs 200a, 200b is connected to an input of the current source 238. The controller 200 can decrease the capacitance of the variable capacitor 180 by increasing the D.C. or low frequency current through the control winding 236. This in turn reduces the permeability of the core 230 (by inhibiting the magnetic domains in the core from following the field fluctuations in the primary winding 232) and hence reduces the inductance presented by the primary winding 232, thereby decreasing the predominance of the capacitive reactance presented by the fixed capacitor 180 over the inductive reactance. Such a change represents an effective decrease in capacitance of the variable capacitor 180. The reverse process produces an increase in capacitance.
One disadvantage of such a device is that it requires a measurement of the forward and reflected voltages and the phase therebetween, or a measurement of the current and voltage and the phase therebetween. Another disadvantage is that it is bulky and costly. Another more important disadvantage is that there are hysteresis losses in each core 234 that vary as the load impedance varies. Referring to FIG. 1B, as the applied magnetic field H (from the control winding 236) increases and then decreases, the induced magnetic field B that fixes the polarization of the core magnetic domains changes at different rates so that there is a net loss of energy with each cycle of the induced field. Referring to FIG. 1C, the complex impedance plane of the match network includes a tuning space 300 within which the impedance match controller provides a theoretically exact solution to the impedance matching problem. Assuming the signal generator output impedance is a purely resistive 50 .OMEGA., if the controller 200 commands a higher control current through the control winding 236, it moves the impedance presented by the pi-network 180, 185, 190 to a lower impedance in the region 310 of the control space. In this case the magnetic core 234 fluctuates near the origin of the hysteresis loop of FIG. 1B and the losses are slight. If on the other hand the controller 200 commands a lower control current, the resulting impedance can be found in the region 320 of higher resistance in the control plane, and the fluctuations in the core 234 may reach the outer extreme of the hysteresis loop of FIG. 1B, in which case the losses in the core 234 are very high. Thus, it is seen that the delivered power to the coil antenna 120 will necessarily vary as the plasma-loaded impedance of the coil antenna 120 varies, a significant disadvantage.
Various other impedance match techniques are known in addition to the foregoing. For example, to avoid the problem of load-dependent losses in the saturable reactor cores, the saturable reactors may be discarded in favor of mechanically variable capacitors. However, such mechanically tunable devices are relatively slow and subject to mechanical breakdown. Frequency tuning is a technique in which the RF generator frequency is varied so as to follow the resonant frequency of the plasma-loaded antenna. The main disadvantage of this technique is that, while it does succeed in maintaining the load impedance nearly free of any reactance, it does result in a variation in delivered RF power as the RF generator frequency is changed, due to impedance magnitude mismatch. Another technique is load power serving, in which the amount of delivered power is monitored, and the RF generator power is varied as necessary so as to maintain the delivered power at a nearly constant level. The disadvantage of this technique is that the power and heat dissipation requirements imposed upon the RF power generating system are greatly expanded, representing an increased cost and bulkiness. Another disadvantage is that it requires the measurement of power, which entails a measurement of voltage, current and the phase between them. The delivered-power servoing technique can be combined with some advantage with the frequency servoing technique.
A more fundamental problem inherent in all of the foregoing conventional techniques is that their controlling parameters (e.g., resonant frequency, delivered power and so forth) are not those directly affecting plasma characteristics controlled by the object (either the overhead coil antenna, electrode or wafer pedestal) to which the RF generator is connected. Specifically, it is the time rate of change of current in the coil antenna which primarily controls plasma density, and it is the wafer voltage which primarily controls ion energy at the wafer. Thus, the foregoing conventional techniques suffer from the disadvantage that they only indirectly maintain a uniform or constant process profile.
It is an object of the invention to tune the RF generator output to the plasma-loaded coil antenna input (load) while avoiding the foregoing problems. Specifically, the problems to be avoided are mechanically adjustable reactance elements (subject to mechanical failure), load-dependent efficiency variations, bulky magnetic components such as saturable reactors, variations in delivered power with changes in load impedance, the necessity of measuring three parameters (e.g., forward and reverse voltages and phase and/or current, voltage and phase).
It is another object of the invention to provide the foregoing tuning function with reference to parameters directly controlling the most important plasma characteristics, including plasma ion density at the wafer surface and plasma ion energy at the wafer surface.